This invention relates to a process for reducing metal containing ions which is employed for preparing a metal supported catalyst by supporting various kinds of catalyst metals onto inorganic supports.
Catalysts comprising inorganic oxide supports such as silica and alumina, and an individual noble metal such as platinum and palladium or the combination of the noble metals, supported thereon have heretofore been employed as those for various chemical reactions and for electrodes of fuel cells. Other catalysts comprising carbon supports and various kinds of catalyst metals such as platinum supported thereon have been also employed.
The catalyst performance of these catalysts depends on the degree of dispersion of the catalyst metals and the performance (specific activity) is promoted with the increase of the surface area of the catalyst if the same amount of the catalyst metals is supported thereon. In preparing the above catalysts, the metal elements are supported onto the inorganic supports by reducing ions containing the catalyst metals to be supported, to the metal elements by means of a reducing agent.
Since, however, such a reducing agent as lithium aluminum hydride and sodium borohydride ordinarily employed is too strong, the particle size of the metals produced by the reduction increases and the particle size distribution becomes broader. In other words, the conventional method has the drawbacks such that the number of the particles decreases to lower the surface area per unit volume of the metal so that the catalytic activity is also made to be lowered, and the particle size becomes considerable lack of uniformity.
Various kinds of alloy catalysts having high catalytic activity have been heretofore proposed after the investigation of the combinations of the supported metals (for example, U.S. Pat. No. 4,447,506).
However, even in these catalysts, the catalyst metals thereof are requested to have a small particle size, that is, a large specific surface area for elevating the activity. Moreover, the alloy catalysts are generally prepared by alloying an alloy component element with a noble metal already supported on the catalyst. It is important from this standpoint to prepare the catalyst supported with a high surface area noble metal having narrow particle size distribution, that is, a uniform particle size.
Having the uniform particle size is important to obtain an alloy catalyst having particles of equal alloy composition.
These catalysts have the drawback that they are likely to be exposed to a high temperature so that the activity may be lost with the lapse of time to shorten the catalyst life if they do not have the resistance to a sintering reaction.